U.N. inspectors in Syria said they were fired upon "deliberately and multiple times" Monday near Damascus as they tried to investigate the scene of a chemical weapons attack that killed hundreds of civilians last week.

The Paris-based group Doctors Without Borders said 355 men, women, and children died of what it called "neurotoxic symptoms" typical in a chemical attack. Thousands more have been treated for the symptoms.

U.S. Secretary of State John Kerry issued the strongest administration statement so far Monday afternoon, calling the attack a "moral obscenity." He accused Syrian President Bashar Assad of destroying the evidence.

Western leaders believe the grudging cooperation by Assad's regime in the probe of the horrific killings is, "too late to be credible."

"The fact is that much of the evidence could have been destroyed by (an) artillery bombardment," British Foreign Secretary William Hague said. "Other evidence could have become degraded over the last few days and other evidence could have been tampered with."

The White House has reluctantly sounded tougher as calls for military action grow. France, Britain, Israel, and some members of Congress are pushing for the United States to get involved.

"This demonstrates yet again that we simply cannot allow the world's most dangerous regimes to acquire the world's most dangerous weapons," Israeli Prime Minister Benjamin Netanyahu, whose military has attacked Syrian targets four times in the past year, said of the chemical attack.

The Israeli newspaper Ha'aretz reported that requests for gas masks in Israel quadrupled over the weekend.

In France, Foreign Minister Laurent Fabius charged Monday that "a chemical massacre happened and the responsibility of Bashar al Assad is clear. There needs to be a reaction."

One U.S. congressman was more specific about what could be done.

"You could destroy (Assad's) munitions and you could destroy his fuel," New York Democrat Eliot Engel suggested on Fox News Sunday. "We could even destroy the Syrian Air Force if we wanted to."

President Barack Obama is still weighing his options. But in a sign the United Sates may be a step closer to an armed response, naval forces have been dispatched toward Syria's coastal waters.

Speaking in South Korea, Sen. John McCain, R-Ariz., cautioned that an ineffective U.S. response would harm American interests globally.

"If the United States stands by and doesn't take very serious action--not just launching some cruise missiles--then again, our credibility in the world is diminished even more, if there's any left," McCain warned.

Meanwhile, Assad enjoys continued backing from Russia, which has been arming Syrian dictators for decades. Foreign Minister Sergei Lavrov blasted Western nations Monday, saying they didn't have evidence Assad is behind the chemical attack.

"They have announced it officially in Washington, London, and Paris that they have incontrovertible evidence regarding the guilt of the Syrian authorities," Lavrov said. "They cannot produce evidence, but keep on saying that the 'red line' has been crossed and they cannot wait any longer."

Assad himself continues to deny responsibility, and he warned that any U.S. intervention would create "a ball of fire that will inflame the Middle East" and result in failure.